The present invention relates generally to exhaust systems for engines and, more particularly, to an exhaust after-treatment system for a low emission, lean-burn internal combustion engine.
Catalysts are generally used as a means for removing pollutants such as HC, CO, and NOx from the exhaust gas of internal combustion engines. The ability of a catalyst to remove NOx in the exhaust gas falls rapidly, however, when the air-fuel ratio of the exhaust gas becomes leaner. Therefore, in engines operating in a lean air-fuel ratio environment, such as stratified charge lean-burn engine, it is difficult to remove NOx from the exhaust gas using a conventional catalyst. Similarly, in the case of diesel engines, a reductant is required to reduce NOx since the exhaust gas has a relatively high concentration of oxygen.
Accordingly, to achieve NOx control, exhaust after-treatment systems have included a NOx conversion device. Presently, however, the performance of NOx conversion technology is limited in several respects. NOx conversion efficiency is affected by the operating temperature of the device, and requires a relatively narrow temperature operating window. In particular, the HC reductant used in diesel engine systems results in a highly exothermic reaction that heats the catalyst. As the catalyst and exhaust gas temperature increases, the HC reductant tends to react more readily with the oxygen than the NOx, thereby reducing the NOx conversion efficiency.
Accordingly, there is a need to maintain the temperature of the exhaust gas within a narrow range such that the HC reductant is more selective towards NOx reduction.
In addition, because the efficiency of catalysts and NOx conversion devices are temperature dependent, a significant portion of harmful emissions can be generated during the cold start portion of the engine cycle, i.e., the first 100-200 seconds after cold start. Thus, there exists a need for an exhaust after-treatment system which provides an effective means of reducing cold start NOx emissions as well.
One object of the present invention is to provide an improved exhaust gas purification system.
The foregoing and other objects and advantages are achieved through the provision of an exhaust gas purification system for a lean-burn engine comprising a thermal mass unit and a NOx conversion catalyst unit downstream of the thermal mass unit. The NOx conversion catalyst unit includes one or more serially arranged catalyst sections which each include a catalytic layer for converting NOx coupled to a heat exchanger. The heat exchanger portion of the catalyst section acts to maintain the catalytic layer substantially at a desired temperature and transfer exothermic heat from the reacting HC reductant of the exhaust gas flowing through the catalytic layer. In one aspect of the invention, each of the several serially arranged catalyst sections are spaced apart from each other within the NOx conversion catalyst unit. This acts to increase the gas turbulence entering each catalyst section thereby increasing NOx conversion efficiency.
In a further aspect of the invention, at least two NOx conversion catalyst units are serially arranged downstream of the thermal mass unit. These additional units can be added as long as there are significant hydrocarbons passed through to the following unit. In such a case, the first NOx conversion catalyst unit obtains a higher NOx conversion efficiency than can be attained with a single NOx conversion catalyst unit.
In a further aspect of the invention, the exhaust gas purification system includes a dual length exhaust pipe including a first path and a second path for. cooling the exhaust gas, and an exhaust valve positioned within the exhaust pipe. The second path can include another heat exchange unit for transferring heat from the exhaust gas flowing through the second path. The exhaust valve controls the mixing of the exhaust flow through the first and second paths to maintain the temperature entering the thermal mass downstream near a desired average temperature.
In another aspect of the invention, a heater is included upstream of the NOx conversion catalyst unit to improve the light-off time of the NOx conversion catalyst unit during engine cold-start.
One advantage of the present invention is that it improves NOx conversion efficiency under transient or steady-state engine operation due to the controlled exhaust gas inlet temperature to the catalysts. Another advantage is that, as compared to conventional catalyst systems, the higher NOx conversion efficiency level results in less HC reductant being used for a given NOx emission reduction level. A further feature of the invention is improved reductions in HC and NOx generated during engine cold start. This results from both the heater and the fact that the catalyst can be located closer to the engine because of the dual length exhaust pipe and heat exchanger.
Other features and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.